This disclosure is directed to a method and apparatus for forming a slug flush and, in particular, a slug flush which has a requisite low density. This is accomplished by foaming a cleaning liquid with air, or some other gas, so that the density of the liquid slug is reduced to accomplish cleaning without destruction which might otherwise occur. More specifically, it is directed to a method for forming a slug of frothed water or other liquid which can be forced through a pipe at high speed for cleaning purposes. It is necessary to periodically clean pipes in refineries, petrochemical processing plants, and the like. It is also necessary to clean such pipes at the time of construction. During the midst of construction, trash and debris accumulates in the pipes. This accumulation of trash arises from several sources. For instance, construction trash will be left in the pipe. In addition to that, the pipe is normally assembled by welding joints and fittings together. This creates welding slag and other construction debris. Often, the pipe is stored in the open before it is assembled in the pipe. In the open, it may accumulate loose dirt, trash, or dried mud on the interior. In addition, small animals will often be caught in the stored pipe or other fittings and may die, leaving a skeleton in the pipe. There are other sources of construction trash and debris which regrettably accumulate, and therefore all of the trash must be removed before the assembled pipe is placed into service. This is true of short as well as long pipes in a petrochemical plant or the like. That is, the pipes are traps for trash which must be cleared before the pipe can be placed in service.
By contrast, a pipe that has been in service for a long period of time may begin with a clean internal wall, but the product which flows through the pipe may form a coating or film on the interior of the pipe. This occurs with the flow of water where any hard minerals in the water adhere to the wall of the pipe and can build up a coating on the interior which can ultimately plug the pipe. Other types of coatings can build up on the interior of the pipe when the product flowing through the pipe is one which is subject to drying or curing. A coating of heavy molecules will coat the pipe line. This is especially true where petroleum products having a wide range of molecular weights flow through the pipe. Sometimes, the heavier greases or sludges will collect and form a film.
Whether it is an original construction problem or a coating that builds up during use, the pipe must be periodically cleaned. One cleaning approach is to force a slug of cleaning fluid through the pipe. The most common process is to force water through the pipe for cleaning. It is not uncommon to position a small nozzle with connected fire hose in a pipe and force it further and further into the line. The jetting action of the water exhausted from the fire hose will, at least to some measure, provide cleaning. This however is considered inadequate for most needs. It has also been common practice to force water through a pipe by delivering water into that line from some kind of storage container. In some instances, the water may be pure and in other instances, the water may have specifically selected additives such as degreasing agents and the like. Typically, that involves the addition of some kind of appropriately selected solvent with the water. In any case, multiple procedures have been implemented in the past to deliver such slugs of fluid through the pipe.
The cleaning process that is contemplated herein is sensitive to the velocity of the flowing slug of cleaning fluid. In the instance of use of a fire hose with a nozzle, cleaning velocities of just a few feet per second can be achieved. These velocities however are significantly inadequate. Other jury rigged procedures can be used which might obtained higher velocities. However, these have their own problems because velocities normally are not high enough. In the event that the velocities are higher, they run the risk of delivering an uncontrolled fluid flush which will damage the pipe and the supporting equipment. For instance, a fire hose might deliver water at about 4 to 8 feet per second. It is possible with connected tanks, with substantially pressure to achieve velocities of 10 or more feet per second. However, and particularly in the case of a larger diameter line (e.g., 6 inches, 16 inches, etc.), the velocity of the slug and the length of the slug become intertwined so that the mechanical dynamics of supporting the pipe become very difficult to handle. A liquid slug of any density whatsoever will create reaction forces as it travels the line. These reaction forces resemble the water hammer which can damage plumbing in residential buildings should the shock wave of the water hammer become excessive. In similar fashion, slug flushing of pipe with a high density liquid creates a risk of damage to the pipe as a result of the reaction forces. The present method and apparatus set forth a system whereby a slug of water or other high density liquid can be delivered; even though high velocities may be achieved for quality cleaning, the density of the slug is reduced and the velocity is controlled so that detrimental mechanical impact forces are avoided in the pipe cleaned by the travelling slug flush. Densities are reduced to perhaps 10 to 25%; most of the slug is air.
One advantage of the present apparatus and method of use is that slug flush cleaning can then be used where the process would otherwise run the risk of creating detrimental water hammer. In particular, a slug of substantial length and velocity is delivered into the pipe. It is however frothed to reduce the density so that the kinetic energy of the travelling slug is not so great that it will damage the pipe when an elbow, tee, or other bend is encountered in the pipe. Consider as an example a slug which is introduced into a pipe having a nominal internal diameter of 12 inches and a length of 1000 feet. Assume further that the velocity of the slug is about 50 l feet per second. This high velocity slug is sufficiently fast for a very good cleaning of on the pipe. The slug will clean practically every type of trash accumulated during construction. Should it be a pipe that has been in serve for some time, the slug will also break up the surface coating and provide a substantial scrubbing to the inside wall of the pipe. In any case, if such a slug were water substantially without bubbles, the froth slug would have substantial kinetic energy. A rough approximation of that kinetic energy for a slug 20 diameters in length in a 12" pipe or 20 cubic feet of water, travelling at 50 feet per second, would deliver sufficient kinetic energy at an elbow in the line that the line would likely be jarred free of structural supports near the elbow; this would damage the line tremendously. If however the density were reduced by 70 to 90 percent, the kinetic energy in the travelling slug would be reduced to levels which may be safely handled by the supports for the line to avoid destruction of the line. In light of this example, the present invention sets forth a method and apparatus enabling delivery of a frothed slug which cleans the line adequately without creating so much kinetic energy in the travelling slug that damage to the line might result. This involves filling a storage tank having a predetermined capacity with air or other driving gas to be delivered behind the slug to propel the slug through the line. It also involves storage of a sufficient amount of liquid in a tank. The preferred liquid for most applications is water, although other liquid solvents can be used. The water is stored in a small tank for delivery into the line to be cleaned. A valve is cracked open at a rate to force the slug of liquid into the line. Delivery of liquid into the line is accomplished simultaneously with delivery of compressed gas into the line; relative diameters of the gas line and the liquid delivery line control the ratio of mixing and to form the froth slug. Gas under pressure is continuously delivered behind the slug to complete the cleaning process.
The apparatus of the present disclosure utilizes a liquid storage tank having two outlet lines. One is from the bottom so that liquid in the tank is delivered through the bottom outlet into the line to be cleaned. Another outlet connects to the top of the tank so that the driving gas is also delivered. These two outlets are relatively sized in accordance with a predetermined ratio so that mixing the liquid and driving gas is assured. A controllable valve connected to a storage tank of predetermined size is switched ratably to deliver the driving gas into the line to force the slug along the line for cleaning purposes. The flow of gas into the storage tank is limited so that pressure peaks or spikes are avoided.